A directional antenna is an antenna which, as a receiving antenna, has a maximum sensitivity in a particular direction. As receiving antennae, directional antennae have a non-isotropic or anisotropic directional characteristic, in other words they have a directional effect. This directional effect can be described quantitatively by the directional factor. The directional characteristic of an antenna can be specified more precisely in an antenna diagram or directional diagram. Similarly to microphones, antennae have different antenna characteristics, for example omnidirectional, cardioid or bidirectional characteristics, as well as lobe characteristics or cardioid-like directional characteristics. In a directional diagram or antenna diagram, directional characteristics of antennae are preferably shown horizontally and vertically in polar coordinates, and specify an angle-dependent antenna gain relative to a maximum signal gain. Directional antennae have a highly anisotropic directional characteristic having a high directional factor, and have a narrow full width at half maximum of the beam angle and a high forward-backward ratio FBR.
Directional antennae can be used in various frequency bands. The configuration and viability of directional antennae depends on the wavelength range used, since the directional characteristic of the directional antenna is dependent on the geometric dimensions of the directional antenna in relation to the wavelength of the received signal.
Directional antennae are formed in particular as portable devices, preferably as portable directional antenna modules, which can be coupled to a receiver. The directional antenna module is guided manually by a user so as to target or locate a signal source, in particular an interfering signal source, in the relevant frequency range. In conventional devices, various directional antenna modules which cover different frequency ranges are mounted for example on a solid handle. By changing out the various directional antenna modules, it is possible to locate signal sources in different frequency ranges. For example, in the conventional Rohde & Schwarz HE300 directional antenna receiver, there is a total of four exchangeable directional antenna modules each having a directional antenna, which together cover a frequency range of 9 kHz to 7.5 GHz. However, it is necessary to change a module of the directional antenna module at a frequency of 20 MHz, 200 MHz and 500 MHz. Since changing out directional antenna modules is relatively laborious for the user to cover a wide frequency range, it has been proposed to accommodate two directional antennae together in a housing and to switch between these directional antennae. FIG. 1 is a simple block diagram of a directional antenna module RA-MOD in which two directional antennae RA are provided in a housing. In the conventional directional antenna module RA-MOD, the two directional antennae RA1, RA2 shown in FIG. 1 are connected to a switch S which can be actuated by a user. In the conventional directional antenna module RA-MOD, a user can switch between the two directional antennae RA1, RA2 by actuating the switch S so as to change the receiving frequency range. For example, in the portable directional antenna module DF-A0047 from Alaris Antennas, the user has to switch manually to the other directional antenna at a frequency of 500 MHz so as to obtain a sufficient signal gain for the antenna received signal.
Operating errors can occur in the aforementioned directional antennae modules. In exchangeable directional antenna modules, it is possible for an incorrect directional antenna for a frequency range to be attached.
The conventional directional antenna module RA-MOD shown in FIG. 1 makes it necessary for the user, in particular in the case of weak received signals, to switch to the other directional antenna manually, starting at the latest from a particular boundary frequency between two adjacent frequency ranges, for example in a narrow overlap range at approximately 500 MHz. The user can derive this instruction for example from a datasheet included with the directional antenna module upon delivery. Thus, one possible source of error during measurement is that the user either does not have access to this information during the measurement process or is not even aware that it is necessary to switch between different directional antennae at a particular boundary frequency. If the user neglects to switch to the other directional antenna, it may therefore occur, in particular for relatively weak received signals, that the measurement using the directional antenna module leads to an excessively weak received signal and the measurement or location thus fails.
Therefore, there is a need to provide a directional antenna module which overcomes the aforementioned drawbacks and makes possible reliable measurement over a total frequency range spanning a plurality of frequency ranges.